有没有办法将 Parse 方法移动到抽象类中?我尝试了多种方法(底部的链接),但我仍然遇到了一个或另一个障碍。
public class AnimalEntityId : EntityId<AnimalEntityId>
{
public AnimalEntityId()
: base()
{
}
private AnimalEntityId(string value)
: base(value)
{
}
public static AnimalEntityId Parse(string value)
{
return new AnimalEntityId(value);
}
}
public abstract class EntityId<TEntityId>
{
private readonly System.Guid value;
protected EntityId(string value)
{
this.value = System.Guid.Parse(value);
}
protected EntityId()
{
this.value = System.Guid.NewGuid();
}
}
尝试了这些建议,但没有运气:
将参数传递给模板化类型的 C# 泛型 new()
C# 中是否有带有参数约束的泛型构造函数?
https://social.msdn.microsoft.com/Forums/en-US/fd43d184-0503-4d4a-850c-999ca58e1444/creating-generic-t-with-new-constraint-that-has-parameters?forum=csharplanguage
- http://www.gamedev.net/topic/577668-c-new-constraint--is-it-possible-to-add-parameters/
提前感谢!
如果你不介意使用反射,你可以像这样Parse
移动到抽象类型中:
public static TEntityId Parse(string val) {
var constr = typeof(TEntityId).GetConstructor(
// Since the constructor is private, you need binding flags
BindingFlags.Instance | BindingFlags.NonPublic
, null
, new[]{ typeof(string) }
, null);
if (constr == null) {
throw new InvalidOperationException("No constructor");
}
return (TEntityId)constr.Invoke(new object[] {val});
}
演示。
不,您不能编写模板约束(例如 new(string)
而不是简单地编写 new()
。你必须利用反射来让它工作:
public abstract class EntityId<TEntityId>
where TEntityId : EntityId<TEntityId>
{
private readonly System.Guid value;
protected EntityId(string value)
{
this.value = System.Guid.Parse(value);
}
protected EntityId()
{
this.value = System.Guid.NewGuid();
}
public static TEntityId Parse(string value)
{
return (TEntityId)Activator.CreateInstance(typeof(TEntityId), new object[] { value });
}
}
假设您使构造函数可访问(而不是当前是私有的)。请注意约束where TEntityId : EntityId<TEntityId>
- 这将确保我们只返回 EntityId
的子类
如何使value
成为私有可变字段/属性并从Parse
方法实际设置它?
(为简单起见,从EntityId
中删除了奇怪的重复通用参数)
public class SimpleAnimalEntityId : EntityId
{
// Implicit parameterless constructor.
}
public class ParametrizedAnimalEntityId : EntityId
{
// Parametrized constructor only.
public ParametrizedAnimalEntityId(int ignored)
{
}
}
public abstract class EntityId
{
// Simple scenario: derived type has a parameterless constructor.
public static TEntity Parse<TEntity>(string value)
where TEntity : EntityId, new()
{
Guid id = Guid.Parse(value);
return new TEntity { value = id };
}
// Advanced scenario: derived type constructor needs parameters injected.
public static TEntity Parse<TEntity>(string value, Func<TEntity> constructor)
where TEntity : EntityId
{
Guid id = Guid.Parse(value);
TEntity entity = constructor();
entity.value = id;
return entity;
}
private Guid value;
protected EntityId()
{
value = Guid.NewGuid();
}
}
现在,您可以从 Parse
方法处理任何构造函数:
string id = Guid.NewGuid().ToString();
SimpleAnimalEntityId simple = EntityId.Parse<SimpleAnimalEntityId>(id);
ParametrizedAnimalEntityId parametrized = EntityId.Parse(id, () => new ParametrizedAnimalEntityId(42));